If your target is a Raspberry Pi image, you can use a pre-defined docker image
instead of installing the toolchain on your local machine. You’ll still need to
create a sysroot folder yourself, with the headers and libraries (see
Making a Raspbian Cross Compilation SDK
for more details).

With the sysroot ready, you can run the following command:

$ docker run -it -v /path/to/sysroot:/rpi/sysroot tritlo/ghc-to-rpi

To launch a docker container where you can cross-compile to your Raspberry Pi by running:

$ arm-linux-gnueabihf-ghc

For easy access, you can add -v$(pwd):/code to the docker launch command to
have the current directory mounted to the /code directory in the container.

Pre-built binary distributions that target iOS (arm64, x86_64), Android (armv7,
arm64, x86_64) as well as Raspberry Pi (armv6) for macOS Sierra and linux (deb8)
can be dowloaded from http://hackage.mobilehaskell.org. Other architectures may
be added at a later date but will for now be built from source. Using hadrian
as the build system is highly recommended.

TODO: Document building from source.

The cross compiler use the LLVM code generator. As such an LLVM installation is
required and needs to be in PATH. The LLVM toolchain provided by Xcode is
insufficient as it does not provide the opt tool.

The pre built GHCs are relocatable, as such it is enough to simply extract
them and add them to the PATH:

$ export PATH=/path/to/ghc-x86_64-apple-ios/bin:$PATH

To provide a unified interface over the cross compilers the general scheme of
the tool chain is $target-tool, e.g. for ghc targeting
aarch64-apple-ios, the tools are:

This unified interface is provided via the toolchain-wrappers for the
pre-built cross compilers. After downloading the toolchain-wrappers, running
the bootstrap.sh script and adjusting the linux-android-toolchain.config
and raspberrypi-toolchain.config files to match the local Android NDK and
Raspberry Pi SDK (TODO: building a raspberry pi SDK), the toolchain should be
usable:

Anything that uses the time package directly or indirectly may unexpectedly
segfault. It appears that the __tzfile_read function from glibc segfaults
when called via tzset from GHC. The exact reasons why this happens are unknown
right now. A suitable workaround is to set the TZ environment variable to a value that’s
not a zonefile when launching the application. E.g. $TZ="UTC0"./Application.